1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus which provides power of an electric motor to the steering system of a vehicle to thereby reduce the manual steering effort or force needed to turn the steering wheel.
2. Description of the Related Art
There are known electric power steering apparatus in which as shown in Japanese Patent Publication No. HEI 7-94227, when an ignition switch is deactivated or switched off during operation of the apparatus, a steering assist force or torque generated by an electric motor is caused to gradually decrease with time.
The conventional electric power steering apparatus is constructed to stop operation of the electric motor to terminate supply of the steering assist force to the steering system when the ignition switch is switched off. Accordingly, if a sensor signal indicative of the off state of the ignition switch is output during movement of the vehicle due to a failure in a sensor provided to detect the on state of the ignition switch, supply of electric power to the electric motor will be stopped to thereby terminate supply of the steering assist force to the steering system even though the vehicle is actually in the running state with the engine operating continuously. Since there is no power assist available from the electric motor, a great muscular effort or force should be exerted by the driver to turn the steering wheel. Additionally, such steering operation is contrary to the driver's anticipative power-assisted steering operation and hence disconcerts the driver. As a result, the steering operation is performed unnaturally and a good steering feel cannot not be obtained.
One solution for the foregoing problems may include an arrangement in which the electric motor can be driven independently from detection of operating condition of the ignition switch. This arrangement, however, has another problem that if the steering wheel is turned repeatedly during stop of the engine with the ignition switch kept in the ON state, the battery will become discharged in a relatively short period of use because the battery power is consumed by the electric motor without being recharged by the alternator which is connected in driven relation to the engine.